Various different gases may be supplied to a process chamber during the formation of a semiconductor or flat panel display device within the chamber. In a chemical vapour deposition process, gases are supplied to a process chamber housing the substrate and react to form a thin film over the surface of the substrate. For example, a LPCVD (low pressure chemical vapour deposition) nitride process uses DCS (dichlorosilane) and ammonia to form silicon nitride on thy surface of a wafer. In an etch process, gases such as boron trichloride and chlorine may be supplied to the chamber to remove unwanted aluminium, and in a polysilicon etch process, hydrogen bromide and chlorine are supplied to the chamber. Cleaning gases such as the perfluorinated compounds CF4, C2F6, NF3 and SF6, and fluorine (F2) may be periodically supplied to the chamber to clean unwanted deposits from the chamber.
A process tool typically has a plurality of process chambers, each of which may be at respective different stage in a deposition, etching or cleaning process, and so the gas being exhausted from the chambers at any given time may have various different pressures, compositions and/or mass flow rates. During these processes, there is typically a residual amount of the gas supplied to the process chamber contained in the gas exhausted from the process chamber. The perfluorinated compounds CF4, C2F6, NF3 and SF6 are known to be greenhouse gases, and so it is desirable to remove these gases from the gas exhausted from the process chambers-prior to the venting of the gas into the atmosphere.
Perfluorinated compounds can be removed from a gas stream with high efficiency using a microwave plasma abatement device. An example of such a device is described in UK Patent no. GB 2,273,027. In that device, a waveguide conveys microwave radiation from a microwave generator into a gas chamber housing two electrodes in a closely opposed relationship. A gas to be treated flows into the gas chamber through a gas inlet, and passes between the electrodes. The electrodes serve to locally enhance the electric field of the microwave radiation passing through the chamber so that a microwave plasma can be initiated and sustained between the two electrodes from the gas flowing between the electrodes. One of the electrodes has an axial hole to provide a gas outlet from the gas chamber. Under the intensive conditions within the plasma, species within the gas stream are subjected to impact with energetic electrons causing dissociation into reactive species that can combine with oxygen or hydrogen added to the gas stream to produce relatively stable by-products.
The destruction and removal efficiency of a microwave plasma abatement device is dependent upon the amount of microwave power that is absorbed by the gas stream flowing through the gas chamber. For any given microwave power, the extent to which the microwave power is absorbed within the chamber is dependent upon a number of factors, including:                chamber pressure;        a the mass flow rate of the gas stream through the chamber;        the composition of the gas stream;        wear or damage to the electrodes or other components of the chamber; and        any debris generated within the chamber from the erosion of the electrodes.        
Therefore, when a single microwave plasma abatement device is arranged to receive the gas exhausted from a plurality of process chambers, it is usual practice to set the power of the microwave radiation at a fixed, relatively high level, for example 6 or 12 kW, in order to ensure that the efficiency of the device remains high at maximum values for the mass flow rate of, and the concentration of perfluorinated compounds within, the gas stream entering the device.
When the concentration of perfluorinated compounds in the gas stream is relatively low, and in particular when the mass flow rate of the gas stream is particularly low, for example when one or more of the process chambers is not being used, generation of the microwave radiation at a relatively high power can result in incomplete absorption of the power of the microwave radiation by the gas flowing within the gas chamber. In addition to the wastage of power, this can result in one or more of the following:                overheating of the gas chamber;        the reflection of microwave radiation back towards the microwave generator, which may result in damage to the microwave generator; and        a change in the impedance of the gas chamber, which may reduce the destruction efficiency of the device.        
It is therefore desirable to minimise the amount of microwave power that is not absorbed within the gas chamber without prejudice to the destruction and removal efficiency of the device.